The basic principles of control of aircraft through coordinated turns has been well understood since it was explained by Orville and Wilbur Wright in their 1906 U.S. Pat. No. 821,393 entitled "Flying Machine". Turning of a modern aircraft is accomplished by actuating small movable panels, or ailerons, on the wing to cause the aircraft to bank, i.e., rotate about its longitudinal axis. The turn is accomplished smoothly, however, only when a degree of rudder control also is imparted. For each aircraft there is a particular amount of rudder control that must be employed for each angle of bank of the aircraft in order to achieve a so-called coordinated turn.
Various instruments have been devised to indicate to the pilot whether or not he is executing a coordinated turn. One of the oldest and most reliable of such instruments is the ball bank instrument which consists of a metal ball inside a liquid filled, curved transparent tube. When the aircraft is not turning or is in a coordinated turn the ball rests in the lowermost, middle region of the curved tube. When the aircraft is in an uncoordinated turn, the ball moves away from the middle of the tube towards one end or the other. During initial training, student pilots are often taught to "kick the ball", i.e., depress the rudder pedal with the foot on the same side of the aircraft as the ball has moved away from the middle of the transparent tube. This rudder movement normally tends to return the aircraft to a coordinated turn condition, regardless of whether it has been in a slip or in a skid.
One of the problems with the ball bank instrument is that it is a visual instrument. To observe it, the pilot must take his eyes away from what is transpiring outside the aircraft. This is undesirable when the aircraft is making turns approaching a landing because the pilot's attention is required outside the aircraft for the purpose of lining up with the landing field and for observing any other aircraft that may be in close proximity. And yet, conditions of the aircraft as it is approaching a landing are such as to demand that well coordinated turns be executed by the pilot. Loss of control of an aircraft due to uncoordinated turns at low speed close to the ground can produce serious and often fatal consequences.
Now, systems have been devised in the past to augment the ball bank instrument in alerting the pilot to undesirable attitudes. For example, U.S. Pat. No. 3,701,092, Hasbrook, granted Oct. 24, 1972 for a "Vehicular Attitude Control Display" discloses a system of gyroscopically controlled indicator lights positioned on the pilot's control wheel for signaling undesirable aircraft attitudes. Although it is speculated that only peripheral vision of the pilot need be employed to read these signal lights, nevertheless, at least some portion of the pilot's visual perceptions are employed, thus distracting his attention from activities outside the aircraft and other visual instruments that must be read.
U.S. Pat. No. 3,089,119, Staples, granted May 7, 1973, entitled "Aural Landing Speed System" and No. 3,924,262, Melancon, granted Dec. 2, 1975 entitled "Aural Warning Apparatus" disclose systems for giving an audio indication to the pilot that the attitude of the aircraft is not correct. Although such systems spare the pilot's visual senses, they can be distracting to his hearing which often upon landing is employed in maintaining contact with the control tower and with other aircraft in the immediate vicinity.
There is thus a need for a simple system for alerting the pilot to an uncoordinated turn and for indicating to the pilot the control corrections required to be made, all without impinging upon his senses of sight and sound so as not to distract these senses from receiving other valuable information that he needs to safely manipulate the aircraft.